Abrasive materials are commonly used to polish surfaces. In the electronics and optics industries, polishing to a very high degree of smoothness is often required. For example, in chip manufacturing thin layers must be removed with extreme precision in a process that eventually yields complex circuitry on a microscopic scale. Increasingly complex, dense function and circuitry requires smaller features (dimensions) of the components on a chip, which, in turn, demands abrasive materials with smaller particle size. While current feature sizes are often less than 0.5 μm, feature sizes under 100 nm, such as 90 nm, 45 nm, 30 nm and less, are envisioned for applications in the near future. References herein to dimensional size, whether with respect to “features” or “diameter,” refer to the smallest dimension of the particle in the sense of the smallest sieve opening that would permit passage of the particle therethrough.
With particular reference to a process known as chemical mechanical planarization (CMP), it is noted that polishing of substrates, which are commonly wafers, is performed with a suspension, dispersion, or slurry of particles. Both tribological and chemical mechanisms are important in producing the finished surface. Parameters such as the erosion rate and the planarity of the substrate are optimized for each application, as described in Chemical-Mechanical Planarization, (Eds. D. S. Boning, K. Devriendt, M. R. Oliver, D. J. Stein, I. Vos; MRS Symp. Proc. V 767; Materials Research Society, 2003); Polishing Surfaces for Integrated Circuits, by B. L. Mueller and J. S. Steckenrider, Chemtech, February, 1998, p 38; and Chemical Mechanical Planarization of Microelectronic Materials, J. M. Steigerwald, S. P. Murarka, R. J. Gutman, Wiley, 1997, all of which are hereby incorporated by reference to the extent relevant and consistent with the discussion herein.
Several examples of making sub-micrometer particles have been described in the literature. See for example, Bickmore et al. U.S. Pat. No. 5,984,997 and references cited therein. Moreover, nanoparticles are well known to the art (e.g. for a review see “Nanoparticles Assemblies and Superstructures, E. N. Kotov, Ed, 2006”).
Martyak et al., U.S. Pat. No. 6,803,353, teach the addition of polishing slurries comprising sulfonated zwitterionic molecules dissolved in the aqueous medium to control the pH. Surfactants are often added to stabilize abrasive nanoparticle suspensions, but because they are surface active they adhere to the nanoparticle by physical forces, which are much weaker than chemical forces. They may also adhere to all surfaces to which they come into contact in the polishing suspension, leaving a residue after the CMP step is completed.
To make a polishing slurry, the particles are dispersed in a liquid, typically water. Many known methods directly produce nanometer colloidal suspensions. For example, Alexander, Iler and Wolter, U.S. Pat. No. 2,601,235, disclose colloidal silica suspensions. Silica colloids are stable suspensions of silica nanoparticles that have a diameter between about 1 nm and about 1 μm. Goodwin, J. W., Colloids and interfaces with surfactants and polymers: an introduction; John Wiley & Sons: Chichester, England, Hoboken, N.J., 2004. Silica colloids serve as inorganic binders for fibers (Fujita, A. U.S. Pat. No. 3,682,668), for stiffeners (Noble, R. D.; Bradstreet, S. W.; Rechter, H. L. U.S. Pat. No. 2,995,453), for refractory coatings (Reuter, R. U.S. Pat. No. 2,856,302), in floor wax to reduce slipping (Iler, R. K. U.S. Pat. No. 2,597,871), for anti-soiling surfaces to reduce pick up of dirt and to leave cleaner appearance after vacuuming (Cogovan, E. J. U.S. Pat. No. 2,622,307), for hydrophilizing surfaces (Blake, R. K. U.S. Pat. No. 3,547,641), and for polishing agents for silicon wafers (Sears, G. W. U.S. Pat. No. 3,922,393). Addition of silica colloids to organic polymers was found to improve durability, adhesion, hardness, and electrical properties of organic coatings. See Cull, N. L. U.S. Pat. No. 3,314,911.
Colloidal dispersions are often referred to as colloidal “solutions” even though they are not true solutions and such terminology is employed herein as well. Any of a variety of methods may be used to produce colloidal silica solutions or “silica sols.” For instance, Alexander, Iler and Wolter, supra, describe the production of 3 weight percent (wt %) silica sols by partially neutralizing a dilute solution of alkali metal silicate with a mineral acid to a pH around 9. In this procedure, sodium silicate solution was first heated at 100° C. for 10 minutes to produce silica nuclei. Then silicate and sulfuric acid solutions were added simultaneously, while vigorously stirring the mixture at 95° C., producing silicic acid. Silicic acid deposited on the nuclei, forming particles with a diameter of 37 nm. Silica sols have been also made from hydrolyzable compounds such as silicon alkoxide, or silicon tetrachloride.
Stöber and Fink (J. Coll. Inter. Sci. 1968, 26, 62) reported that monodisperse suspensions of silica particles could be prepared by hydrolyzing a lower alkyl silicate in an alcohol medium in the presence of suitable amounts of water and ammonia. The particle diameter ranged from 0.05 to 2.0 μm. The chemistry is thought to involve the condensation of intermediate Si—OH groups to yield Si—O—Si networks. Other approaches to making stable silica sols include, electrodialysis, ion exchange, and dissolution of elemental silicon.
Stöber and Fink, supra, also reported that monodisperse suspensions of silica particles could be prepared by hydrolyzing a lower alkyl silicate in an alcohol medium in the presence of suitable amounts of water and ammonia. The particle diameter ranged from 0.05 to 2.0 μm. The chemistry is thought to involve the condensation of intermediate Si—OH groups to yield Si—O—Si networks. Other approaches to making stable silica sols include, electrodialysis (e.g. Sanchez, Canadian Patent 586,261), ion exchange (e.g. Bird, P. G. U.S. Pat. No. 2,244,325), and dissolution of elemental silicon (e.g. Balthis, J. H. U.S. Pat. No. 2,614,994)
Two of the problems commonly faced by those preparing and using suspensions or slurries of nanoparticles for polishing are the tendency toward agglomeration and the difficulty in removing particulates after CMP. With respect to the tendency of particles to agglomerate, it has been noted that silica sol will aggregate irreversibly by changing the ionic strength, concentration, pH, temperature, or addition of oppositely charged macromolecules and incompatible organic solvents. Aggregation broadly describes the different ways in which silica particles are precipitated out of the solution. It includes gelling, coagulation and flocculation.
Various approaches exist to stabilize nanoparticle suspensions against agglomeration or aggregation. Often, nanoparticles possess charged surfaces causing the particles to be electrostatically repelled from each other. In particular, stability is an important issue in formulations comprising silica sols, which derive their stability from electrostatic repulsion. As the solution pH increases above 2, the protonated silanol groups, Si—OH, start to dissociate, forming SiO-ions. At a certain point, sufficient negative ionic charges develop on the surface and mutual repulsion ensues.
It is well known that stable suspensions or colloids of well dispersed particles may be achieved. Unfortunately, however, these suspensions are destabilized in the presence of salts of sufficient ionic strength because the salt ions screen the electrostatic repulsions, allowing the particles to come into contact. Once in contact, different short-range forces take over and the particles remain adhered. The effective particle size is thus increased and the polishing performance of the suspension is negatively impacted.
Surfactants are often added to polishing suspensions to assist in the dispersal of the nanoparticles. However, surfactants adhere to all solid/aqueous interfaces and after the CMP step in chip manufacturing the surfactants must be removed before the next step. They become, in effect, unwanted residue.
With respect to the problem associated with removal of particulates following CMP, it has been found that particles adhere to a surface by van der Waals forces, electrostatic forces, capillary forces (arising from the surface tension of the liquid drawn into the capillary spaces around the contact points) and the like. As a general rule, the smaller the particle the more difficult it is to entirely remove particles from a surface. Thus, it is especially challenging to completely remove nanoparticles from a surface. Misra et al. U.S. Pat. No. 7,087,564, and references therein, describe methods to remove residues of polishing slurries in the “post-CMP cleaning” process.
Therefore, there is a need for abrasive nanoparticles that can be employed in polishing, which form stable suspensions under a wide range of conditions and which demonstrate minimal adhesion to surfaces.
Other problems encountered during the manufacture of silica colloid products include ensuring the absence of flocculating agents, controlling the pH, controlling temperature, and regulating ionic strength. These complications limit the scope of practical uses for silica colloid. Thus, there is a need to enhance the stability of colloidal silica dispersions.
Surface modification of silica colloids has been employed to alleviate instability. Alexander and Iler, exchanged aluminates into the SiO2 surface, creating aluminosilicate sites having a fixed negative charge irrespective of the solution pH. The modified sols were more stable toward gelling at low pH and were less sensitive to salt than unmodified silica sols. One of the shortcomings, of aluminosilicate modified silica sols is their instability in solutions with ionic strength that exceeds 0.3 M NaCl, especially at pH 8.5, where they exist in a fully negatively charged state. It was also noted that aluminosilicate modified silica sols coagulated in the presence of proteins such as albumin bovine, possibly, due to ionic interactions with cationic groups on the protein surface.
In an alternative approach, Alexander and Bolt mixed acidified silica sols with oxides of aluminum, gallium, titanium or zirconium. The positively charged metal salts adsorbed to the silica surface, and reversed its charge. These modified silica colloids lack surface silanol groups, and siloxane bonds do not form when the sol is dried to a powder. As a result, these powders can be redispersed to a sol in water at pH 3-5. However the leaching of Al3+ from the surface or the presence of polyvalent anions shows a destabilizing effect, especially at low pH.
To supplement ionic stabilization, the idea of steric hindrance was introduced. This was accomplished via adsorption of excess positively charged polymer on the silica surface. The length of the polymer chains is often smaller than the size of the silica colloid, and flocculation through interparticle bridging does not take place.
In addition to the conventional “homonucleus” silica sols described above, silica colloidal formulations with core-shell morphology have been also prepared. The surface properties of these colloids are governed by the silica shell, while the core comprises a different material that imparts the particles with desirable optical, magnetic, or catalytic properties. The core can be a modified silane reagent, or metal colloids such as silver and gold. Wiesner and Ow disclosed a process to prepare nanoparticle compositions with a core comprising fluorescent silane compounds and a silica shell on the core. The core was prepared by reacting 3-aminopropyltriethoxy silane with either tetramethylrhodamine isothiocyanate or ALEXA FLOUR® dyes. The product was hydrolyzed in a mixture of water, ammonia and alcohol, using a modified version of the Stöber method. Tetraethoxy silane was then added to the core containing solution and formed the silica shell upon hydrolysis.